According to this news item,
Shadow justice secretary Richard Burgon said he did not trust Boris Johnson not to call an election for mid-October and then change the date afterwards.
This implies the PM can actually change the date. However, as far as I understand the Fixed-term Parliaments Act 2011, it's the Queen who sets the date:
the polling day for the election is to be the day appointed by Her Majesty by proclamation on the recommendation of the Prime Minister
So all this "change the date" could only happen if Boris Johnson asks the Queen to proclaim election for e.g. 15th October, then ask her again to proclaim election for e.g. 1st November. As far as I know, in the past century the Queen (and previous Kings) didn't really go against the recommendations of their respective PMs - but this "let's change the date of the already proclaimed election" trickery seems to be a great opportunity to do so. Is it a real possibility that the PM can change the date of an election?